Credit Losses on Mortgage-Backed Securities for which Portion of Other-Than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 442	$ 425
Reductions for securities sold during the period	(442)	(85)
Additional credit losses for which other-than-temporary impairment charge was previously recorded		102
Ending balance		$ 442